A conventional GSM network is schematically illustrated in FIG. 1. A mobile switching centre (MSC) 2 is connected via communication links to a number of base station controller (BSCs) 4. The BSCs 4 are dispersed geographically across areas served by the mobile switching centre 2. Each BSC 4 controls one or more base transceiver stations (BTSs) 6 located remote from, and connected by further communication links to, the BSC 4. Each BTS 6 transmits radio signals to, and receives radio signals from, mobile stations 8 which are in an area served by that BTS 6. That area is referred to as a “cell”. A GSM network is provided with a large number of such cells, which are ideally contiguous to provide continuous coverage over the whole network territory.
A mobile switching centre 2 is also connected via communication links to other mobile switching centres in the remainder of the mobile communications system 10, and to a public service telephone network (PSTN), which is not illustrated. The mobile switching centre 2 is provided with a home location register (HLR) 12 which is a database storing subscriber authentication data including the international mobile subscriber identity (IMSI) which is unique to each mobile station 8. The IMSI is also stored in the mobile station in a subscriber identity module (SIM) along with other subscriber-specific information.
The mobile switching centre 2 is also provided with a visitor location register (VLR) 14 which is a database temporarily storing subscriber authentication data for mobile stations which are active in the area served by the mobile switching centre 2.
The mobile switching centre 2 may also be provided with a subscriber location register (SLR) 18 which is a database storing data relating the HLRs and VPSs of the network with the network's subscribers.
The GSM network supports a variety of services. One such service is an answerphone service. The answerphone service, among other services, is implemented on a Voice Processing System (VPS) 16. The VPS 16 is linked to the mobile switching centre 2. Incoming calls to the MSC 2 which are not answered by a called subscriber are automatically forwarded to the VPS 16. FIG. 1 only illustrates a single VPS 16. However, a mobile communications network generally comprises several VPSs 16 each serving a plurality of subscribers. Various implementations of the answerphone service are possible, of which one example is described below.
In the implementation which this invention concerns, the answerphone service is accessed by means of one or more directory numbers which may be called by all, or at least a plurality of, subscribers, and possibly non-subscribers, to access the service (hereinafter referred to as “common telephone numbers of the answerphone service”). For a subscriber, the common number may be dialled manually, or generated by the handset itself on the subscriber pressing a “dial answerphone option”.
In other implementations, each subscriber is provided with two directory numbers, one identifying the subscriber's handset and the other identifying the subscriber's mailbox. If a call is made to the number identifying the subscribers mailbox, the subscriber is allowed to access their messages, if any, sometimes subject to entry of a personal PIN code for the subscriber.
The answerphone service provides a personal mailbox for each subscriber to the answerphone service. When a call is forwarded to a subscribers mailbox, a personal message of the subscriber is played to the caller. The caller may then leave a voice message in the mailbox. The subscriber is notified by a GSM short message that a message has been deposited in the subscriber's mailbox. The subscriber can then access his/her mailbox in order to retrieve the message.
A call to the subscriber which is not answered is automatically forwarded to the answerphone service and the called subscriber's telephone number is used to access the subscriber's mailbox. However, a call to the subscriber may not only be forwarded if the called subscriber is busy, out of coverage or does not answer, but also if the subscriber's elects, via an option on the handset, to have the call forwarded to his mailbox. Such a call is referred to as a conditionally diverted call. The subscriber can elect which calls, i.e. the calls from which callers, are to be diverted. The subscriber sets on the handset a telephone number where a call is to be diverted to. That telephone number may be a telephone number which subscribers commonly use to access the answerphone service.
The directory number (MSISDN) of the subscriber to which the call is directed is transmitted as the CLI (calling line identity) on diversion of a call from the subscriber's handset. The CLI, being unique for each subscriber, can be used by the VPS to access the subscriber's mailbox.
If a subscriber wants to access the answerphone service in order to retrieve a message from his/her mailbox, the subscriber dials the common telephone number of the answerphone service on his/her handset. The retrieval call can be automatically forwarded to the subscriber's mailbox by using the CLI. An incoming retrieval call can be distinguished from a conditionally diverted call by the lack of a divert flag which is set on diversion and transmitted to the answerphone service.
If a subscriber wants to access his/her mailbox from abroad, i.e. outside the coverage area of the home network (referred to as a “roaming” subscriber), the call is directed to the answerphone service via the visited network. However, no CLI is transmitted and the subscriber is not forwarded automatically to his/her mailbox. Rather, the mailbox number has to be entered manually to identify the mailbox from which a call is to be retrieved.